1. Field of the Invention
The present invention relates to a method for forming a chromium oxide film on the surface of stainless steel, and, more particularly, to a method for forming a chromium oxide film, as a passivation layer, on a stainless-steel surface, by which oxidation resistance is markedly increased with reduced moisture adsorption, and diffusion and permeation of hydrogen into the stainless steel can be sharply prevented.
2. Description of the Related Art
The definition of the terms xe2x80x9cclean surfacexe2x80x9d in the vacuum related fields varies according to vacuum exposure environments. In other words, xe2x80x9ccleanxe2x80x9d means much more than scrubbing the sample and handling it with care. For example, in an ultra high vacuum of 1xc3x9710xe2x88x929 Torr or an extreme high vacuum of 1xc3x9710xe2x88x9212 Torr, a xe2x80x9cclean surfacexe2x80x9d is defined as a surface at which outgassing due to thermal effects does not occur beyond a particular level. For the reduction of outgassing, any ultra high vacuum chamber and the compartments thereof must be subjected to pretreatment, such as chemical cleaning or electrolytic polishing.
Stainless steels are the preferred materials for ultra high vacuum or extreme high vacuum processing conditions because of their superior oxidation resistance, low outgassing rate, and easy welding properties.
Stainless steels have a native passivation oxide layer. Although the surface of stainless steel is protected by the native passivation oxide layer, it still has a strong affinity for gases, so that when exposed to air, the surface is prone to absorb gases such as water vapor. Water molecules are adsorbed onto the surface or into the near surface region of stainless steel, and the porous surface oxide layer serves as a reservoir for water. This weakness of stainless steel against moisture sorption and subsequent outgassing has been a problem in unbaked stainless steel vacuum systems.
The conventional surface treatment technique can create an ultra high vacuum condition to a certain extent. However, since the hydrophilic porous surface absorbs excess water, it takes a long time to evacuate the chamber and the degree of vacuum is also lowered.
To solve the above problems, it is an objective of the present invention to provide a method for processing the surface of stainless steel, by which moisture sorption, and diffusion and permeation of hydrogen can be suppressed, so that evacuation time can be sharply reduced with an improved degree of vacuum.
To achieve the above objective of the present invention, there is provided a method for forming a chromium oxide film on a stainless steel surface, comprising: (a) placing a sample having the stainless steel surface into a vacuum furnace, evacuating the vacuum furnace to a pressure of 2xc3x9710xe2x88x927 to 3xc3x9710xe2x88x927 Torr, and heating the vacuum furnace to 450 to 600xc2x0 C. at a rate of 5 to 10xc2x0 C./min; (b) maintaining the pressure in the vacuum furnace for 10 to 20 minutes at a temperature of 450 to 600xc2x0 C. to remove foreign materials from the surface of the stainless steel and to diffuse chromium atoms from the interior of the stainless steel; and (c) supplying oxygen into the vacuum furnace while maintaining the pressure and temperature until an oxygen partial pressure reaches 1xc3x9710xe2x88x929 to 2.5xc3x9710xe2x88x927 Torr, to cause the diffused chromium atoms to react with oxygen, resulting in the chromium oxide (Cr2O3) film on the surface of the stainless steel.
Preferably, step (c) is carried out for 50 seconds to 28 hours.
Preferably, when the temperature of the vacuum furnace is 450xc2x0 C., step (c) is carried out at a pressure of 1xc3x9710xe2x88x929 to 2xc3x9710xe2x88x929 Torr for 14 to 28 hours.
Preferably, when the temperature of the vacuum furnace is 500xc2x0 C., step (c) is carried out at a pressure of 8xc3x9710xe2x88x929 to 9xc3x9710xe2x88x929 Torr for 3 to 3.5 hours.
Preferably, when the temperature of the vacuum furnace is 550xc2x0 C., step (c) is carried out at a pressure of 5xc3x9710xe2x88x928 to 6xc3x9710xe2x88x928 Torr for 1,600 to 2,000 seconds. Preferably, when the temperature of the vacuum furnace is 600xc2x0 C., step (c) is carried out at a pressure of 2.5xc3x9710xe2x88x927 to 3.5xc3x9710xe2x88x927 Torr for 300 to 400 seconds.
Preferably, the stainless steel includes 304, 304L, 316, 316L and 316LN stainless steels.